Systems and methods including multi-mode operation of optically pumped magnetometer(s)

ABSTRACT

A magnetic field measurement system that includes at least one magnetometer; at least one magnetic field generator; a processor coupled to the at least one magnetometer and the at least one magnetic field generator and configured to: measure an ambient background magnetic field using at least one of the at least one magnetometer in a first mode selected from a scalar mode or a vector mode; generate, in response to the measurement of the ambient background magnetic field, a compensation field using the at least one magnetic field generator; and measure a target magnetic field using at least one of the at least one magnetometer in a spin exchange relaxation free (SERF) mode which is different from the first mode.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.16/814,926, filed Mar. 10, 2020, which is a divisional of U.S. patentapplication Ser. No. 16/213,980, filed Dec. 7, 2018, which issued asU.S. Pat. No. 10,627,460, which claims the benefit of U.S. ProvisionalPatent Application Ser. No. 62/723,933, filed Aug. 28, 2018, all ofwhich are incorporated herein by reference in their entirety.

FIELD

The present disclosure is directed to the area of magnetic fieldmeasurement systems using one or more optically pumped magnetometers.The present disclosure is also directed to magnetic field measurementsystems and methods that include operation in scalar/vector and spinexchange relaxation free (SERF) modes using one or more magnetometers.

BACKGROUND

In the nervous system, neurons propagate signals via action potentials.These are brief electric currents which flow down the length of a neuroncausing chemical transmitters to be released at a synapse. Thetime-varying electrical current within the neuron generates a magneticfield, which propagates through the human body and can be measured usingeither a Superconductive Quantum Interference Device (SQUID) or anOptically Pumped Magnetometer (OPM). In this disclosure the OPM isprimarily considered because the SQUID requires cryogenic cooling, whichmay make it prohibitively costly for users and too large to be wearableby a user. In addition to OPMs and SQUIDs, other magnetic sensingtechnologies for detection of magnetic fields from the brain include andmagnetoresistance.

Optical magnetometry can include the use of optical methods to measure amagnetic field with very high accuracy—on the order of 1×10⁻¹⁵ Tesla. Ofparticular interest for their high-sensitivity, an optically pumpedmagnetometer (OPM) can be used in optical magnetometry to measure weakmagnetic fields. In at least some embodiments, the OPM has an alkalivapor gas cell that contains alkali metal atoms in a combination of gas,liquid, or solid states (depending on temperature). The gas cell maycontain a quenching gas, buffer gas, or specialized antirelaxationcoatings or any combination thereof. The size of the gas cells can varyfrom a fraction of a millimeter up to several centimeters.

BRIEF SUMMARY

One embodiment is a magnetic field measurement system that includes atleast one magnetometer; at least one magnetic field generator; aprocessor coupled to the at least one magnetometer and the at least onemagnetic field generator and configured to: i) measure an ambientbackground magnetic field using at least one of the at least onemagnetometer in a first mode selected from a scalar mode or a vectormode; ii) generate, in response to the measurement of the ambientbackground magnetic field, a compensation field using the at least onemagnetic field generator; iii) measure a target magnetic field using atleast one of the at least one magnetometer in a spin exchange relaxationfree (SERF) mode which is different from the first mode and iv)determine when the at least one of the at least one magnetometer is notoperating in SERF mode and automatically perform steps i) and ii) again.For example, the first mode can be a scalar mode or a non-SERF vectormode.

In at least some embodiments, the at least one magnetometer includes afirst magnetometer configured to operate in both the first mode and theSERF mode and the processor is configured to operate the firstmagnetometer in both the first mode and the SERF mode. In at least someembodiments, the at least one magnetometer includes a first magnetometerconfigured to operate in the first mode and a second magnetometerconfigured to operate in the SERF mode. In at least some embodiments,each of the at least one magnetometer includes a vapor cell and thefirst and second magnetometers share the same vapor cell.

In at least some embodiments, the processor is further configured to:measure the ambient background magnetic field, reduced by compensationfield, using at least one of the at least one magnetometer in the SERFmode; and update, in response to the measurement of the ambientbackground magnetic field reduced by compensation field, thecompensation field using the at least one magnetic field generator. Inat least some embodiments, measuring the ambient background magneticfield includes, for each of two or three orthogonal axes: applying afirst magnetic field along the axis; sweeping a frequency of the firstmagnetic field; measuring responses by the at least one of the at leastone magnetometer in the first mode during the sweeping; and determininga vector component of the ambient background magnetic field along theaxis by observing a maximum or minimum in the responses or fitting theresponses to a Lorentzian function.

In at least some embodiments, measuring the ambient background magneticfield includes applying a first magnetic field along a first axis;sweeping a frequency of the first magnetic field; measuring responses bythe at least one of the at least one magnetometer in the first modeduring the sweeping; and determining a magnitude of the ambientbackground magnetic field along the axis by observing a maximum orminimum in the responses or fitting the responses to a Lorentzianfunction. In at least some embodiments, measuring the ambient backgroundmagnetic field further includes, for each of two or three orthogonalaxes: applying a first magnetic field along the axis; modulating thefirst magnetic field; measuring responses by the at least one of the atleast one magnetometer in a vector mode during the modulation; anddetermining a vector component of the ambient background magnetic fieldalong the axis by observing the responses to the modulated firstmagnetic field.

In at least some embodiments, the magnetic field measurement systemfurther includes at least one local oscillator coupled to the at leastone magnetometer; and at least one lock-in amplifier, each of the atleast one lock-in amplifier coupled to a one of the at least one localoscillator and at least one of the at least one magnetometer.

Another embodiment is a magnetic field measurement system that includesat least one first magnetometer configured for operation in a first modeselected from a scalar mode or a vector mode; at least one secondmagnetometer configured for operation in a spin exchange relaxation free(SERF) mode which is different from the first mode; at least onemagnetic field generator; and a processor coupled to the at least onefirst magnetometer, the at least one second magnetometer, and the atleast one magnetic field generator and configured to: measure an ambientbackground magnetic field using at least one of the at least one firstmagnetometer; and generate of a compensation field by the at least onemagnetic field generator based on the measurement from the at least onefirst magnetometer. For example, the first mode can be a scalar mode ora non-SERF vector mode.

In at least some embodiments, each of the at least one firstmagnetometer and each of the at least one second magnetometer includes avapor cell and at least one of the at least one first magnetometer andat least one of the at least one second magnetometers share the samevapor cell.

In at least some embodiments, the processor is further configured to:measure the ambient background magnetic field, reduced by compensationfield, using at least one of the at least one second magnetometer; andupdate, in response to the measurement of the ambient backgroundmagnetic field reduced by compensation field, the compensation fieldusing the at least one magnetic field generator. In at least someembodiments, measuring the ambient background magnetic field includes,for each of two or three orthogonal axes: applying a first magneticfield along the axis; sweeping a frequency of the first magnetic field;measuring responses by the at least one of the at least one firstmagnetometer in the first mode during the sweeping; and determining avector component of the ambient background magnetic field along the axisby observing a maximum or minimum in the responses or fitting theresponses to a Lorentzian function.

In at least some embodiments, measuring the ambient background magneticfield includes: applying a first magnetic field along a first axis;sweeping a frequency of the first magnetic field; measuring responses bythe at least one of the at least one first magnetometer in the firstmode during the sweeping; and determining a magnitude of the ambientbackground magnetic field along the axis by observing a maximum orminimum in the responses or fitting the responses to a Lorentzianfunction. In at least some embodiments, measuring the ambient backgroundmagnetic field further includes, for each of two or three orthogonalaxes: applying a first magnetic field along the axis; modulating thefirst magnetic field; measuring responses by the at least one of the atleast one first or second magnetometer in a vector mode during themodulation; and determining a vector component of the ambient backgroundmagnetic field along the axis by observing the responses to themodulated first magnetic field.

In at least some embodiments, the magnetic field measurement systemfurther includes at least one local oscillator coupled to at least oneof the at least one first magnetometer; and at least one lock-inamplifier, each of the at least one lock-in amplifier coupled to a oneof the at least one local oscillator and at least one of the at leastone first magnetometer.

A further embodiment is a non-transitory processor readable storagemedia that includes instructions for operating a magnetic fieldmeasurement system including at least one magnetometer and at least onemagnetic field generator, wherein execution of the instructions by oneor more processors cause the one or more processors to perform actions,including: i) measuring an ambient background magnetic field using atleast one of the at least one magnetometer operating in a first modeselected from a scalar mode or a vector mode; ii) generating, inresponse to the measurement of the ambient background magnetic field, acompensation field using the at least one magnetic field generator; iii)measuring a target magnetic field using at least one of the at least onemagnetometer operating in a spin exchange relaxation free (SERF) modewhich is different from the first mode and iv) determining when the atleast one of the at least one magnetometer is not operating in SERF modeand automatically performing steps i) and ii) again. For example, thefirst mode can be a scalar mode or a non-SERF vector mode.

In at least some embodiments, measuring the ambient background magneticfield using the at least one of the at least one magnetometer operatingin a first mode and measuring the target magnetic field using at leastone of the at least one magnetometer operating in the spin exchangerelaxation free (SERF) mode include measuring the ambient backgroundmagnetic field and measuring the target magnetic field using the samemagnetometer. In at least some embodiments, the actions further include:measuring the ambient background magnetic field, reduced by compensationfield, using at least one of the at least one magnetometer in the SERFmode; and updating, in response to the measurement of the ambientbackground magnetic field reduced by compensation field, thecompensation field using the at least one magnetic field generator.

In at least some embodiments, measuring the ambient background magneticfield includes, for each of two or three orthogonal axes: applying afirst magnetic field along the axis; sweeping a frequency of the firstmagnetic field; measuring responses by the at least one of the at leastone magnetometer in the first mode during the sweeping; and determininga vector component of the ambient background magnetic field along theaxis by observing a maximum or minimum in the responses or fitting theresponses to a Lorentzian function.

In at least some embodiments, measuring the ambient background magneticfield includes: applying a first magnetic field along a first axis;sweeping a frequency of the first magnetic field; measuring responses bythe at least one of the at least one magnetometer in the first modeduring the sweeping; and determining a magnitude of the ambientbackground magnetic field along the axis by observing a maximum orminimum in the responses or fitting the responses to a Lorentzianfunction. In at least some embodiments, measuring the ambient backgroundmagnetic field further includes, for each of two or three orthogonalaxes: applying a first magnetic field along the axis; modulating thefirst magnetic field; measuring responses by the at least one of the atleast one magnetometer in a vector mode during the modulation; anddetermining a vector component of the ambient background magnetic fieldalong the axis by observing the responses to the modulated firstmagnetic field.

Yet another embodiment is a method of operating a magnetic fieldmeasurement system that includes at least one magnetometer and at leastone magnetic field generator. The method includes i) measuring anambient background magnetic field using at least one of the at least onemagnetometer operating in a first mode selected from a scalar mode or avector mode; ii) generating, in response to the measurement of theambient background magnetic field, a compensation field using the atleast one magnetic field generator; iii) measuring a target magneticfield using at least one of the at least one magnetometer operating in aspin exchange relaxation free (SERF) mode which is different from thefirst mode; and iv) determining when the at least one of the at leastone magnetometer is not operating in SERF mode and performing steps i)and ii) again. For example, the first mode can be a scalar mode or anon-SERF vector mode.

In at least some embodiments, measuring the ambient background magneticfield using the at least one of the at least one magnetometer operatingin the first mode and measuring the target magnetic field using at leastone of the at least one magnetometer operating in the spin exchangerelaxation free (SERF) mode include measuring the ambient backgroundmagnetic field and measuring the target magnetic field using the samemagnetometer.

In at least some embodiments, measuring the ambient background magneticfield using the at least one of the at least one magnetometer operatingin a first mode includes measuring the ambient background magnetic fieldusing a first magnetometer of the at least one magnetometer, the firstmagnetometer operating in the first mode; and measuring the targetmagnetic field using at least one of the at least one magnetometeroperating in the spin exchange relaxation free (SERF) mode includemeasuring the target magnetic field using a second magnetometer of theat least one magnetometer, the first magnetometer operating in the SERFmode. In at least some embodiments, each of the at least onemagnetometer includes a vapor cell and the first and secondmagnetometers share the same vapor cell.

In at least some embodiments, the method further includes measuring theambient background magnetic field, reduced by compensation field, usingat least one of the at least one magnetometer in the SERF mode; andupdating, in response to the measurement of the ambient backgroundmagnetic field reduced by compensation field, the compensation fieldusing the at least one magnetic field generator. In at least someembodiments, measuring the ambient background magnetic field includes,for each of two or three orthogonal axes: applying a first magneticfield along the axis; sweeping a frequency of the first magnetic field;measuring responses by the at least one of the at least one magnetometerin the first mode during the sweeping; and determining a vectorcomponent of the ambient background magnetic field along the axis byobserving a maximum or minimum in the responses or fitting the responsesto a Lorentzian function.

In at least some embodiments, measuring the ambient background magneticfield includes: applying a first magnetic field along a first axis;sweeping a frequency of the first magnetic field; measuring responses bythe at least one of the at least one magnetometer in the first modeduring the sweeping; and determining a magnitude of the ambientbackground magnetic field along the axis by observing a maximum orminimum in the responses or fitting the responses to a Lorentzianfunction. In at least some embodiments, measuring the ambient backgroundmagnetic field further includes, for each of two or three orthogonalaxes: applying a first magnetic field along the axis; modulating thefirst magnetic field; measuring responses by the at least one of the atleast one magnetometer in a vector mode during the modulation; anddetermining a vector component of the ambient background magnetic fieldalong the axis by observing the responses to the modulated firstmagnetic field.

BRIEF DESCRIPTION OF THE DRAWINGS

Non-limiting and non-exhaustive embodiments of the present invention aredescribed with reference to the following drawings. In the drawings,like reference numerals refer to like parts throughout the variousfigures unless otherwise specified.

For a better understanding of the present invention, reference will bemade to the following Detailed Description, which is to be read inassociation with the accompanying drawings, wherein:

FIG. 1A is a schematic block diagram of one embodiment of a magneticfield measurement system, according to the invention;

FIG. 1B is a schematic block diagram of one embodiment of amagnetometer, according to the invention;

FIG. 1C is a schematic block diagram of one embodiment of opticalelement of a magnetometer, according to the invention;

FIG. 2 shows a magnetic spectrum with lines indicating dynamic ranges ofmagnetometers operating in different modes;

FIG. 3A is a schematic view of one embodiment of a dual arrangement of aSERF mode magnetometer and a scalar mode magnetometer, according to theinvention;

FIG. 3B is a schematic view of another embodiment of a dual arrangementof a SERF mode magnetometer and a scalar mode magnetometer, according tothe invention;

FIG. 3C is a schematic view of a vapor cell of yet another embodiment ofa dual arrangement of a SERF mode magnetometer and a scalar modemagnetometer, according to the invention

FIG. 4 is a flowchart of one embodiment of method of operating amagnetic field measurement system, according to the invention;

FIG. 5 is a flowchart of another embodiment of method of operating amagnetic field measurement system, according to the invention;

FIG. 6 is a schematic view of yet another embodiment of a dualarrangement of a SERF mode magnetometer and a scalar mode magnetometeralong with graphs showing a resulting measured magnetic field withsweeping of applied magnetic fields along the respective axes, accordingto the invention;

FIG. 7A is a schematic view of one embodiment of an arrangement of amagnetometer and an auxiliary sensor, according to the invention;

FIG. 7B is a schematic view of another embodiment of an arrangement of amagnetometer and an auxiliary sensor, according to the invention;

FIG. 8 is a flowchart of one embodiment of method of determining acompensation field, according to the invention;

FIG. 9 is a schematic view of one embodiment of an arrangement of amagnetometer, magnetic field generators, a local oscillator, and alock-in amplifier, according to the invention;

FIG. 10 is a flowchart of another embodiment of method of determining acompensation field, according to the invention;

FIG. 11 illustrates a top graph of measured detector voltage versus timeand a bottom graph of drive frequency versus time, according to theinvention;

FIG. 12 illustrates graphs of in-phase (“U”), quadrature (“V”), andphase angle (“θ”) outputs of a lock-in amplifier during sweeping of afrequency of an applied magnetic field, according to the invention; and

FIG. 13 is a schematic view of one embodiment of an arrangement of amagnetometer, magnetic field generators, three local oscillators, andthree lock-in amplifiers, according to the invention.

DETAILED DESCRIPTION

The present disclosure is directed to the area of magnetic fieldmeasurement systems using one or more optically pumped magnetometers.The present disclosure is also directed to magnetic field measurementsystems and methods that include operation in scalar/vector and spinexchange relaxation free (SERF) modes using one or more magnetometers.

Herein the terms “ambient background magnetic field” and “backgroundmagnetic field” are interchangeable and used to identify the magneticfield or fields associated with sources other than the magnetic fieldmeasurement system and the biological source(s) (for example, neuralsignals from a user's brain) or other source(s) of interest. The termscan include, for example, the Earth's magnetic field, as well asmagnetic fields from magnets, electromagnets, electrical devices, andother signal or field generators in the environment, except for themagnetic field generator(s) that are part of the magnetic fieldmeasurement system.

The terms “gas cell”, “vapor cell”, and “vapor gas cell” are usedinterchangeably herein. Below, a gas cell containing alkali metal vaporis described, but it will be recognized that other gas cells can containdifferent gases or vapors for operation.

An optically pumped magnetometer (OPM) is a basic component used inoptical magnetometry to measure magnetic fields. While there are manytypes of OPMs, in general magnetometers operate in two modalities:vector mode and scalar mode. In vector mode, the OPM can measure one,two, or all three vector components of the magnetic field; while inscalar mode the OPM can measure the total magnitude of the magneticfield.

Vector mode magnetometers measure a specific component of the magneticfield, such as the radial and tangential components of magnetic fieldswith respect the scalp of the human head. Vector mode OPMs often operateat zero-fields and may utilize a spin exchange relaxation free (SERF)mode to reach femto-Tesla sensitivities. A SERF mode OPM is one exampleof a vector mode OPM, but other vector mode OPMs can be used at highermagnetic fields. These SERF mode magnetometers can have high sensitivitybut in general cannot function in the presence of magnetic fields higherthan the linewidth of the magnetic resonance of the atoms of about 10nT, which is much smaller than the magnetic field strength generated bythe Earth. As a result, conventional SERF mode magnetometers oftenoperate inside magnetically shielded rooms that isolate the sensor fromambient magnetic fields including Earth's.

Magnetometers operating in the scalar mode can measure the totalmagnitude of the magnetic field. (Magnetometers in the vector mode canalso be used for magnitude measurements.) Scalar mode OPMs often havelower sensitivity than SERF mode OPMs. However, scalar mode OPMs canoperate in unshielded environments up to and including the Earth field,which is about 50 μT. Furthermore, as the magnetic readings from scalarmode OPMs do not suffer from long-term drifts and bias they arefrequently used to calibrate other magnetic sensors.

In optically pumped magnetometers (OPMs), which are based on theprecession of atomic spins, another classification is based on thestrength of the effective magnetic field experienced by the atoms in thegas cell, where two regimes are identified: zero-field mode andfinite-field mode. Finite-field OPMs operate in a regime where themagnitude of the field experienced by the atoms is much larger than thewidth of their magnetic resonance. Examples of finite-field OPMs includeboth scalar and vector mode magnetometers in driven, relaxation, andfree-induction decay modalities.

Zero-field OPMs operate in an effective magnetic field whose strength issmaller, or comparable, to the linewidth of the magnetic resonance ofthe atoms. It will be understood that a zero-field OPM need not operatein strictly zero magnetic field, but rather in a relatively low magneticfield as described in the preceding sentence. Examples of zero-fieldmagnetometers include OPMs operating in SERF mode in either DC ormodulated schemes. Zero-field magnetometers typically measure one or twovector components of the field and are among the most sensitivemagnetometers to date. However, as their operation requires a lowmagnetic field environment, they are usually deployed inside expensive,bulky, and sophisticated magnetically shielded rooms.

In any OPM mode (SERF, vector, or scalar) magnetic noise should beconsidered. For instance, in one specific application of OPMs thatinvolves measuring magnetic signals from the brain (i.e.magnetoencephalography or MEG), magnetic noise arises from oscillationsof the magnetic field which have the same frequencies as neural signalsand can overwhelm the magnetic signals of the brain. If these signalsoriginate far from the region of interest (e.g., the human brain) thenthey can be suppressed by sampling and then subtracting the backgroundfield measured by a combination of two sensors. This technique is calledgradiometry. First order gradiometer uses two sensors, second orderthree sensors, and so on. The higher the order the better background issuppressed but results in a more complicated system with many sensorsthat just measure background signal and don't contribute to themeasurement of brain signal.

Conventional SERF mode systems have often used vapor cell magnetometersin combination with fluxgate or magnetoresistive magnetometers as a wayto reach the SERF regime. Such implementations may use readings from theauxiliary sensor (for example, a fluxgate or magnetoresistive device) aserror signals that are passed to magnetic coils, on a continuous orperiodic or aperiodic basis, to modify or null the ambient backgroundmagnetic field at the position of the SERF mode magnetometer. Objectivesof this active-shielding technique can include any of the following: i)suppression of the static and slowly varying components of the ambientbackground magnetic field so that the SERF mode magnetometer can operatewithin its dynamic range; ii) mitigation of spurious fast-varying fieldsthat, while not bringing the SERF mode magnetometer outside its dynamicrange, can be confounded with the target signal; and iii) activesuppression of 60 Hz or 50 Hz power line noise that radiates from allalternating current power lines. The difference between 60 Hz and 50 Hzdepends on the region of the world where this device is used. NorthAmerica is 60 Hz while Europe and parts of Asia use 50 Hz. There can bechallenges that may limit the performance and versatility of these SERFmode systems, such as, for example, poor common-mode background fieldrejection ratio due to the use of devices placed far apart from eachother (for example, a few centimeters apart) such as in the use of abulky SERF mode magnetometer and a bulky auxiliary sensor; and thelimitation of the SERF mode magnetometer by intrinsic performance of theauxiliary sensor including, for example, (a) intrinsic noise of theauxiliary sensor (which can range, for example, from 1 pT/sqrt(Hz) forfluxgates to hundreds of pT/sqrt(Hz) for magnetoresistance devices, andis at least 1 to 3 orders of magnitude higher than what is required forMEG detection) which is translated to magnetic noise by a feedback loopor (b) the intrinsic offset of the auxiliary sensor (which may be of theorder of 10 nT for both fluxgates and magnetoresistance devices and canbe outside of the dynamic range of SERF mode magnetometers) which istranslated to magnetic offset by a feedback loop.

In contrast to these conventional systems, systems and methods aredescribed herein that combine SERF mode operation of an optically pumpedmagnetometer (OPM) with scalar or non-SERF vector mode magnetic fieldsensing using the same or a different OPM. This system and methods, inat least some embodiments, can enable, for example, wearablemagnetoencephalography (MEG) sensing systems.

The term “non-SERF vector mode”, as utilized to describe methods,systems, and other embodiments of the invention, will refer to amagnetometer operating in any vector mode other than the SERF mode.

A magnetic field measurement system, as described herein, can includeone or more (for example, an array of) optically pumped magnetometers.In at least some embodiments, as described herein, the system can bearranged so that at least one (or even all) of the magnetometers can beoperated sequentially in i) the scalar or non-SERF vector mode and ii)the SERF mode. In at least some embodiments, the system can be arrangedso that at least one of the magnetometers can be operated in the scalaror non-SERF vector mode and at least one of the magnetometers can beoperated in the SERF mode. In at least some of these embodiments, ascalar or non-SERF vector mode magnetometer and a SERF mode magnetometermay utilize the same vapor cell, as described below.

The magnetic field measurement systems described herein can be used tomeasure or observe electromagnetic signals generated by one or moresources (for example, biological sources). The system can measurebiologically generated magnetic fields and, at least in someembodiments, can measure biologically generated magnetic fields in anunshielded or partially shielded environment. Aspects of a magneticfield measurement system will be exemplified below using magneticsignals from the brain of a user; however, biological signals from otherareas of the body, as well as non-biological signals, can be measuredusing the system. Uses for this technology outside biomedical sensinginclude, but are not limited to, navigation, mineral exploration,non-destructive testing, detection of underground devices, asteroidmining, and space applications. In at least some embodiments, the systemcan be a wearable MEG system that can be used outside a magneticallyshielded room.

FIG. 1A is a block diagram of components of one embodiment of a magneticfield measurement system 140. The system 140 can include a computingdevice 150 or any other similar device that includes a processor 152 anda memory 154, a display 156, an input device 158, one or moremagnetometers 160, one or more magnetic field generators 162, and,optionally, one or more sensors 164. The system 140 and its use andoperation will be described herein with respect to the measurement ofneural signals arising from signal sources in the brain of a user as anexample. It will be understood, however, that the system can be adaptedand used to measure other neural signals, other biological signals, aswell as non-biological signals.

The computing device 150 can be a computer, tablet, mobile device, fieldprogrammable gate array (FPGA), microcontroller, or any other suitabledevice for processing information. The computing device 150 can be localto the user or can include components that are non-local to the userincluding one or both of the processor 152 or memory 154 (or portionsthereof). For example, in at least some embodiments, the user mayoperate a terminal that is connected to a non-local computing device. Inother embodiments, the memory 154 can be non-local to the user.

The computing device 150 can utilize any suitable processor 152including one or more hardware processors that may be local to the useror non-local to the user or other components of the computing device.The processor 152 is configured to execute instructions, as describedbelow.

Any suitable memory 154 can be used for the computing device 150. Thememory 154 illustrates a type of computer-readable media, namelycomputer-readable storage media. Computer-readable storage media mayinclude, but is not limited to, volatile, nonvolatile, non-transitory,removable, and non-removable media implemented in any method ortechnology for storage of information, such as computer readableinstructions, data structures, program modules, or other data. Examplesof computer-readable storage media include RAM, ROM, EEPROM, flashmemory, or other memory technology, CD-ROM, digital versatile disks(“DVD”) or other optical storage, magnetic cassettes, magnetic tape,magnetic disk storage or other magnetic storage devices, or any othermedium which can be used to store the desired information and which canbe accessed by a computing device.

Communication methods provide another type of computer readable media;namely communication media. Communication media typically embodiescomputer-readable instructions, data structures, program modules, orother data in a modulated data signal such as a carrier wave, datasignal, or other transport mechanism and include any informationdelivery media. The terms “modulated data signal,” and “carrier-wavesignal” includes a signal that has one or more of its characteristicsset or changed in such a manner as to encode information, instructions,data, and the like, in the signal. By way of example, communicationmedia includes wired media such as twisted pair, coaxial cable, fiberoptics, wave guides, and other wired media and wireless media such asacoustic, RF, infrared, and other wireless media.

The display 156 can be any suitable display device, such as a monitor,screen, or the like, and can include a printer. In some embodiments, thedisplay is optional. In some embodiments, the display 156 may beintegrated into a single unit with the computing device 150, such as atablet, smart phone, or smart watch. In at least some embodiments, thedisplay is not local to the user. The input device 158 can be, forexample, a keyboard, mouse, touch screen, track ball, joystick, voicerecognition system, or any combination thereof, or the like. In at leastsome embodiments, the input device is not local to the user.

The magnetic field generator(s) 162 can be, for example, Helmholtzcoils, solenoid coils, planar coils, saddle coils, electromagnets,permanent magnets, or any other suitable arrangement for generating amagnetic field. The optional sensor(s) 164 can include, but are notlimited to, one or more magnetic field sensors, position sensors,orientation sensors, accelerometers, image recorders, or the like or anycombination thereof.

The one or more magnetometers 160 can be any suitable magnetometerincluding, but not limited to, any suitable optically pumpedmagnetometer. In at least some embodiments, at least one of the one ormore magnetometers (or all of the magnetometers) of the system isarranged for operation in both i) the scalar or non-SERF vector mode andii) the SERF mode. Alternatively or additionally, the one or moremagnetometers 160 of the system include at least one scalar or non-SERFvector mode magnetometer and at least one SERF mode magnetometer.Examples of dual mode systems are disclosed in U.S. Patent ProvisionalPatent Application Ser. No. 62/723,933, incorporated herein by referencein its entirety.

As a further example of an optically pumped magnetometer that canoperate in both i) the scalar or non-SERF vector mode and ii) the SERFmode, an alkali metal magnetometer can be operated as a zero-fieldmagnetometer with the ability to operate in SERF mode with suppressedspin-exchange relaxation. At finite magnetic fields, such that theLarmor precession frequency is much higher than the intrinsic spinrelaxation, the same magnetometer can be used to measure the magnitudeof the magnetic field when the magnetometer is operating in the scalarmode (or the non-SERF vector mode).

FIG. 1B is a schematic block diagram of one embodiment of a magnetometer160 which includes an alkali metal gas cell 170 (also referred to as a“cell”); a heating device 176 to heat the cell 170; a light source 172;and a detector 174. The gas cell 170 can include, for example, an alkalimetal vapor (for example, rubidium in natural abundance, isotopicallyenriched rubidium, potassium, or cesium, or any other suitable alkalimetal such as lithium, sodium, or francium), quenching gas (for example,nitrogen) and buffer gas (for example, nitrogen, helium, neon, orargon). The light source 172 can include, for example, a laser tooptically pump the alkali metal atoms and to probe the gas cell, as wellas optics (such as lenses, waveplates, collimators, polarizers, andobjects with reflective surfaces) for beam shaping and polarizationcontrol and for directing the light from the light source to the celland detector. The detector 174 can include, for example, an opticaldetector to measure the optical properties of the transmitted lightfield amplitude, phase, or polarization, as quantified through opticalabsorption and dispersion curves, spectrum, or polarization or the likeor any combination thereof.

In a scalar mode magnetometer (e.g., an optically pumped magnetometeroperating in the scalar mode), in addition to the above elements, alocal oscillator (LO) (see, for example, FIG. 9) is added to drive thespin precession on resonance with the Larmor frequency as set by thegiven ambient field. The excitation can be introduced in the form of anRF field generated using the magnetic field generator 162 or opticallyby modulating the intensity, frequency, or polarization of the pumpinglight beam from the light source 172.

Examples of suitable light sources include, but are not limited to, adiode laser (such as a vertical-cavity surface-emitting laser (VCSEL),distributed Bragg reflector laser (DBR), or distributed feedback laser(DFB)), light-emitting diode (LED), lamp, or any other suitable lightsource. Examples of suitable detectors include, but are not limited to,a photodiode, charge coupled device (CCD) array, CMOS array, camera,photodiode array, single photon avalanche diode (SPAD) array, avalanchephotodiode (APD) array, or any other suitable optical sensor array thatcan measure the change in transmitted light at the optical wavelengthsof interest.

Examples of magnetic field measurement systems or methods of making suchsystems or components for such systems are described in U.S. ProvisionalPatent Application Ser. Nos. 62/689,696; 62/699,596; 62/719,471;62/719,475; 62/719,928; 62/723,933; 62/732,327; 62/732,791; 62/741,777;62/743,343; 62/745,144; 62/747,924; and 62/752,067, all of which areincorporated herein by reference in their entireties.

FIG. 1C illustrates one example of am optically pumped magnetometer(OPM) 160 that includes a light source 172, a converging lens 178, awave retarder 180, a vapor cell 170, and a detector 174. The lightsource 172 (for example, a distributed feedback laser (DFB), verticalcavity surface-emitting laser (VCSEL), or edge emitting laser diode)radiates light at its intrinsic beam divergence angle. The converginglens 178 can be used to collimate the light into a parallel path. Thelight is transformed from linearly to circularly polarized via the waveretarder 180 (for example, a quarter wave plate). The light passesthrough the alkali metal vapor in the gas cell 170 and is rotated orabsorbed by the alkali metal vapor before being received by the detector174.

FIG. 2 shows the magnetic spectrum from 1 fT to 100 μT in magnetic fieldstrength on a logarithmic scale. The magnitude of magnetic fieldsgenerated by the human brain are indicated by range 201 and themagnitude of the background ambient magnetic field, including theEarth's magnetic field, by range 202. The strength of the Earth'smagnetic field covers a range as it depends on the position on the Earthas well as the materials of the surrounding environment where themagnetic field is measured. Range 210 indicates the approximatemeasurement range of a magnetometer operating in the SERF mode (e.g., aSERF magnetometer) and range 211 indicates the approximate measurementrange of a magnetometer operating in the scalar mode (e.g., a scalarmagnetometer.) Typically, a SERF magnetometer is more sensitive than ascalar magnetometer but many conventional SERF magnetometers typicallyonly operate up to about 20 to 200 nT while the scalar magnetometerstarts in the 100 fT range but extends above 10 μT. At very highmagnetic fields the scalar magnetometer typically becomes nonlinear dueto a nonlinear Zeeman splitting of atomic energy levels.

FIG. 3A illustrates a dual arrangement with a SERF mode magnetometer 360a and a scalar or non-SERF vector mode magnetometer 360 b in combinationwith a magnetic field generator 362. As an example, this dualarrangement could be placed on the scalp 330 of a user to measure MEG.In at least some embodiments, the magnetometers 360 a, 360 b are at thesame location as the two magnetometers both use the same alkali vaporcell or the two magnetometers are actually the same magnetometeroperating sequentially in i) the scalar or non-SERF vector mode and ii)the SERF mode. In other embodiments, the magnetometers 360 a, 360 b areseparate from each other, but are preferably located close together.

FIG. 3B illustrates a dual arrangement of SERF mode magnetometer 360 aand a scalar or non-SERF vector mode magnetometer 360 b in combinationwith a magnetic field generator 362 that is placed around a portion ofthe user's head 330.

As described in more detail below, the dual arrangement of a SERF modemagnetometer 360 a and a scalar or non-SERF vector mode magnetometer 360b can include, but is not limited to, a) a single magnetometer thatalternates operation in i) the SERF mode and ii) the scalar or non-SERFvector mode or b) an arrangement using a single vapor cell with a firstportion of the vapor cell operating as a SERF mode magnetometer and asecond portion of the vapor cell operation as a scalar or non-SERFvector mode magnetometer. Examples of the later arrangement can be foundin U.S. Provisional Patent Application Ser. Nos. 62/699,596 and62/732,327, both of which are incorporated herein by reference in theirentireties. FIG. 3C also illustrates a vapor cell 370 where a portion ofthe vapor cell forms a SERF mode magnetometer 360 a because the ambientbackground magnetic field at that portion has been reduced sufficientlythat the portion can operate in the SERF mode. Other portions of thevapor cell 370 can operate as a scalar or non-SERF vector modemagnetometer 360 b. In other embodiments, the magnetometers 360 a, 360 bare separate from each other, but are preferably located close together.

The systems and methods as described herein utilize active shielding byemploying the magnetic field generators 362. It will be understood,however, that the systems and methods may also include passive shieldingusing materials such as mu-metal and ferrite or any other suitablecomponents. Examples of passive shielding are found in U.S. ProvisionalPatent Application Ser. Nos. 62/719,928 and 62/752,067, all of which areincorporated herein by reference in their entireties.

FIG. 4 illustrates one embodiment of a method of operation or use of amagnetic field measurement system that contains a dual arrangement of aSERF mode magnetometer and a scalar or non-SERF vector modemagnetometer, as well as active shielding using a magnetic fieldgenerator or the like, as illustrated in FIGS. 3A and 3B. In step 402,the ambient background magnetic field is measured using the magnetometeroperating in a first mode selected from a scalar mode or a non-SERFvector mode. In step 404, this ambient background field is canceled, orreduced substantially (e.g., by at least 80, 90, 95, or 99 percent) bythe application of a compensation field (i.e., a local field) using themagnetic field generator. The compensation field may be, for example,equal and opposite to the ambient background magnetic field or at least80, 90, 95, or 99 percent of the ambient background magnetic field.

In step 406, the system can measure the vector components of themagnetic field due to neural activity (or any other target magneticfield of interest) using a magnetometer operating in a SERF modedifferent from the first mode. In step 408, the system or userdetermines whether to continue measuring. If no, then the system endsoperation. If yes, then in step 410 the system determines whether themagnetometer continues to operate in SERF mode. If yes, the systemreturns to step 406 to make another measurement. If no, such as when thecompensation field is no longer sufficient to reduce the ambientbackground magnetic field to a magnitude that allows the magnetometer tooperate in SERF mode (for example, 200 nT, 50 nT, 20 nT, or less), thenthe system returns to step 402 to measure the ambient backgroundmagnetic field and, in step 404, modify or otherwise alter thecompensation field.

As an example of step 410, in at least some embodiments, the system isconfigured to determine when SERF mode is lost (for example, bycomparing transmitted light level as measured by the detector withrespect to a threshold value or flatness of response. The SERF mode maybe lost when, for example, the ambient background magnetic fieldundergoes a rapid change in amplitude or direction (or both).

As an alternative to step 410, the system may automatically switch fromthe SERF mode to the scalar or non-SERF vector mode periodically (forexample, at a specific or selected repetition rate) or aperiodically toperform steps 402 and 404 again. In some embodiments, this switchingbetween modes may occur at least every 0.5, 1, 2, 5, 10, 50, 100, or 500milliseconds or every 0.5, 1, 2, 5, 10, or 30 seconds or every 1, 2, 5,or 10 minutes.

FIG. 5 illustrates another embodiment of a method of operation or use ofa magnetic field measurement system that contains a dual arrangement ofa SERF mode magnetometer and a scalar or non-SERF vector modemagnetometer, as well as active shielding using a magnetic fieldgenerator or the like, as illustrated in FIGS. 3A and 3B. In step 502,the ambient background magnetic field is measured using the magnetometeroperating in a first mode selected from a scalar mode or a non-SERFvector mode. In step 504, this ambient background field is canceled, orreduced substantially (e.g., by at least 80, 90, 95, or 99 percent) bythe application of a compensation field (i.e., a local field) using themagnetic field generator. The compensation field may be, for example,equal and opposite to the ambient background magnetic field or at least80, 90, 95, or 99 percent of the ambient background magnetic field.

In step 506, a second measurement of the ambient background magneticfield (as reduced by the compensation field—i.e., the reduced ambientbackground magnetic field) is performed using a magnetometer operatingin a SERF mode different from the first mode. In step 508, in responseto this second measurement, the compensation field can be altered orupdated to ‘fine-tune’ the cancelation or reduction of the ambientbackground magnetic field.

In step 510, the system can measure the vector components of themagnetic field due to neural activity (or any other target magneticfield of interest) using the magnetometer operating in the SERF mode. Instep 512, the system or user determines whether to continue measuring.If no, then the system ends operation. If yes, then in step 514 thesystem determines whether the magnetometer continues to operate in SERFmode. If yes, the system returns to step 510 to make another measurementor, optionally (indicated by the dotted line in FIG. 5), the systemreturns to step 506 to again measure the reduced ambient backgroundmagnetic field with the magnetometer in SERF mode to update thecompensation field. If the magnetometer is no longer operating in SERFmode in step 514, such as when the compensation field is no longersufficient to reduce the ambient background magnetic field to amagnitude that allows the magnetometer to operate in SERF mode (forexample, 200 nT, 50 nT, 20 nT, or less), then the system returns to step502 to measure the ambient background magnetic field and, in step 504,modify or otherwise alter the compensation field.

One embodiment of a magnetic field measurement system that includes anoptically pumped magnetometer 160 (FIG. 1A) that can operate in both i)SERF mode and ii) scalar or non-SERF vector mode at interleaved periodsof time using the same vapor cell 170 (FIG. 1B). This approach has theadvantage of in situ-magnetometry and compensation or, in other words,the ambient background magnetic field is measured at the same locationas the magnetometer that measures the biological or other signal ofinterest. Alternatively, the magnetic field measurement system includesa vapor cell that can be used for both a SERF mode magnetometer and ascalar or non-SERF vector mode magnetometer as illustrated in FIG. 3C.In other embodiments, the magnetometers are separate from each other,but are preferably located close together.

In at least some embodiments, these magnetic field measurement systemscan operate according to either of the methods illustrated in FIG. 4 or5, as described above. For example, when the user initially starts upthe system (for example, a wearable MEG system) the ambient backgroundmagnetic field is measured using a magnetometer in scalar or non-SERFvector mode. Next, the system applies a compensation field using themagnetic field generator (for example, active shielding electromagnetssuch as Helmholtz coils.) Then the system switches to SERF mode andoptionally performs a measurement of the reduced ambient backgroundmagnetic field. In at least some embodiments, this high-accuracymeasurement allows the system to update the cancelation field tocompensate for small changes in the ambient background magnetic field.

With the system operating in SERF mode, the magnetometer then measuresneural activity or other biosignal or signal of interest. In at leastsome embodiments, the system continues to measure the signal of interestuntil the compensation field no longer sufficiently reduces the ambientbackground magnetic field so that the SERF mode is disrupted. Then thesystem switches back to using scalar or non-SERF vector magnetometry toagain measure the ambient background magnetic field and adjust or modifythe compensation field so that SERF mode operation is again possible. Inat least some embodiments, if the background field drifts slowly theshift will be measured in SERF mode and the compensation coils updatedaccordingly without leaving SERF mode.

Referring now to FIG. 6, the system (or any other system describedherein) can utilize a dual OPM arrangement 660 a, 660 b (an arrangementof one or more magnetometers which can operate in i) SERF mode and ii)scalar or non-SERF vector mode, as described above) surrounded by threepairs of compensation coils 662 a, 662 b, 662 c which can generate thethree components, Bx′, By′, Bz′, respectively, of the compensationmagnetic field to cancel or reduce the ambient background magnetic field(including the Earth's magnetic field). When in the scalar mode, the OPMonly measures the magnitude of the ambient background magnetic field butit cannot directly determine the individual three vector components. Inother words, the scalar magnetometer measures (Bx²+By²+Bz²)^(1/2) whereBx, By and Bz are the three Cartesian components of the ambientbackground magnetic field. To individually measure the three vectorcomponents, the magnetic field produced by the three pairs ofcompensation coils 662 a, 662 b, 662 c can be individually swept to findthe minimum of the resulting field, as illustrated in graphs 690, 691,692, which corresponds to sweeping through the three vector components,Bx′, By′, Bz′, respectively. Each of the graphs 690, 691, 692 illustratethe measured field magnitude versus the swept vector component with theminimum corresponding to the swept vector component approximatelyequaling the corresponding vector component of the ambient backgroundmagnetic field.

In another embodiment, the scalar mode magnetometer measures themagnitude of the ambient background magnetic field,|B|=(Bx²+By²+Bz²)^(1/2) where Bx, By and Bz are the three Cartesiancomponents of the ambient background magnetic field. The magnetic fieldvector generated by the system using the magnetic field generator isgiven by: B′=xBx′+yBy′+zBz′ where x, y and z are Cartesian unit vectors.In a configuration where the laser is oriented along the x-axis of thesystem, to measure the By component of the background field, the y-axismagnetic field, By′, is swept over a range of values, for example, −10μT to 10 μT and |B| is measured. |β|=(Bx+(By²+By′²)+Bz²)^(1/2). Theoutput of the detector is a Lorentzian function with a peak at By′=−By.By^(C) is set at this value. This procedure can be repeated to find Bz.Bx is found by first zeroing By and Bz by applying By^(C) and Bz^(C)then sweeping Bx′ over the range of interest. When By′=−By the output ofthe detector reaches a minimum in an inverted Lorentzian. Bx^(C) is setto the value of Bx′ when the minimum occurs. In some instances, anadditional oscillating component added to Bx′ may be employed toincrease the sensitivity of accuracy of the By^(C) and Bz^(C)measurements by narrowing the Lorentzian response.

In other embodiments, the system includes an optically pumpedmagnetometer that can be operated in SERF mode, an optically pumpedmagnetometer (the same or different from the first magnetometer) thatcan be operated in the scalar or non-SERF vector mode, and one or moreauxiliary sensors, such as a fluxgate or magnetoresistance device. In atleast some embodiments, the one or more auxiliary sensors can beoperated concurrently, or at interleaved periods of time with themagnetometer(s). In at least some embodiments, the one or more auxiliarysensors can be operated continually or periodically. In at least someembodiments, the one or more auxiliary sensors can be used to measurethe ambient background magnetic field with these measurements can beused to produce the compensation field and the magnetometer operating inthe scalar or non-SERF vector mode can be used from time to time torecalibrate the one or more auxiliary sensors. This calibration isuseful because the one or more auxiliary sensors are located at adistance from the magnetometer operating in the SERF mode, whereas themagnetometer operating in the scalar or non-SERF vector mode can be thesame magnetometer or use the same vapor cell or located near themagnetometer operating in SERF mode. In at least some embodiments, anadvantage of the approach is that measurements with the auxiliarysensors may be faster, thus reducing the time between consecutive SERFmeasurements and measurement bandwidths.

FIGS. 7A and 7B illustrate two embodiments of arrangements that aresimilar to those illustrated in FIGS. 3A and 3B, respectively, exceptthat these arrangements include an auxiliary sensor 364 such as amagnetoresistance device or fluxgate sensor, in addition to themagnetometer(s) 360 and magnetic field generator 162 of FIG. 1A.

There are a variety of methods and techniques for determining theambient background magnetic field and setting a compensation field. FIG.8 illustrates one embodiment of such a method which can, at least insome embodiments, be used for steps 402 and 404 in FIG. 4 or steps 502and 504 in FIG. 5.

In at least some embodiments, this method uses intensity measurementsfrom an optically pumped magnetometer operating with a single pump/probelaser as the light source and with a single photodiode as the detector(although other light sources and detectors can be used.). When thetotal magnetic field is aligned with the pump axis (in this case,designated to be the x-axis) the measured optical signal intensity ismaximum. Accordingly, if any component of the magnetic field appearsalong the y or z axes, the measured optical transmitted intensity (TI)will be reduced. It will be understood that the x, y, and z axesdescribed below are interchangeable meaning that the x axis can bereplaced by the y or z axes and so on.

The ambient background magnetic field can be determined by varying theapplied fields By′ and Bz′ until the total intensity is maximum. Whenthis occurs By′ ˜−By and Bz′˜−Bz

Turning to FIG. 8, in step 802 a large field, Bx^(L), is applied by thesystem where Bx^(L) is larger than the background Bx to ensure Bx+Bx^(L)is not zero. For example, if Bx could be any value from −5 μT to 5 μTthen adding a Bx^(L) of 10 uT would results in a non-zero range from 5μT to 15 μT.

In step 804, By′ is swept along the range of possible By. If the rangeof expected By were from −5 μT to 5 μT then By′ might be swept from −6μT to 6 μT. In step 806, the transmitted intensity (TI) is monitoredduring the sweep and the maximum is found when By′˜−By. By^(C) is thenset to −By, the value that provided the maximum TI. When this is donethe total field along the y-axis is zero; By+By^(C)=0.

In step 808 Bz′ is swept over the expected range of Bz. In step 810, thetransmitted intensity (TI) is monitored during the sweep and the maximumis found when Bz′˜−Bz. Once the maximum TI value is found Bz^(C) is setto −Bz. When this is done, Bz+Bz^(C)=0. If completed, there is noremaining magnetic field along the y or z axes.

In steps 812 to 816, Bx is determined. In step 812, a large backgroundfield, By^(L), is applied along the y-axis (or equivalently on thez-axis), similar to step 802, and then in step 814 Bx′ is swept over theexpected range. In step 816, the transmitted intensity (TI) is monitoredduring the sweep and the maximum is found when Bx′˜−Bx. Once the maximumTI value is found Bx^(C) is set to −Bx. When this is done, The totalmagnetic field along the x-axis is zero: Bx+Bx^(C)=0.

Using this procedure in steps 802 to 816, the compensation fieldsBx^(C), By^(C), Bz^(C) are determined and can be applied using themagnetic field generators with the result that the total remainingmagnetic fields from the combination of the background field the andcorrection fields along each Cartesian axis are all equal zero or arenear zero (for example, a reduction of at least 80, 90, 95, or 99percent in the ambient background magnetic field.)

Other embodiments for determining a compensation field utilize a lock-inamplifier. FIG. 9 illustrates one embodiment of a portion of a magneticfield measurement system 940 with a computing device 950 (designated asthe “OPM control electronics”), a magnetometer 960, and a magnetic fieldgenerator 962 disposed around the magnetometer. The magnetic fieldgenerator 962 includes three pairs of coils (labeled Bx, By, and Bz) andarranged to produce three fields Bx′, By′, and Bz′. The embodimentillustrated in FIG. 9 has a local oscillator (LO_x) 982 applied to thecoils Bx so that the coils Bx can generate an RF field with a frequencyω_(x) can be swept as described below. The embodiment illustrated inFIG. 9 also includes a lock-in amplifier (LIA_x) 984 that receives inputfrom the detector of the magnetometer 960 and the local oscillator 982and can be used to provide in-phase (“U”), quadrature (“V”), and phaseangle (“θ”) outputs based on the inputs, as described in more detailbelow.

In at least some embodiments, the systems or methods may utilizetechniques, such as, but not limited to, least square fitting,filtering, and machine learning, to infer magnetic fields based onsensor outputs and field-generator inputs.

FIG. 10 illustrates an embodiment of such a method which can, at leastin some instances, be used for steps 402 and 404 in FIG. 4 or steps 502and 504 in FIG. 5. First, a magnetometer operating in scalar mode isused to determine the magnitude |β| of the ambient background magneticfield. FIG. 9, described above, illustrates one arrangement for makingsuch a measurement. The process can be thought of as using a drivenoscillator for measuring the resonance frequency ω₀ of the oscillator,also known as Larmor frequency, that is directly related to themagnitude of the magnetic field B by the gyromagnetic ratio γ of theatomic species used in the OPM according to Equation 2:

ω₀ =γ|B|  2)

As an example, an OPM is placed inside a passive magnetic shield. Thepassive shield attenuates the ambient background magnetic field at theposition of the OPM by a factor of, for example, 500 to 1000.Alternatively, similar measurements can be performed in an unshieldedenvironment or in a partially shielded environment with shieldingfactors ranging from 10 to 500. In step 1002, to estimate |B| using theOPM the motion of the spins is driven using an oscillating magneticfield B_(mod)(t)=B_(m) cos(ω_(m) t) {circumflex over (x)} where ω_(m) isgenerated using a local oscillator (for example, LO_x 982 of FIG. 9). Instep 1004, the frequency ω_(m) is swept (using, for example, a linearchirp) while the absorption of the transmitted light is monitored by thedetector. One example of the result is illustrated in the top graph inFIG. 11 which graphs the detector voltage versus the scan time. Thebottom graph of FIG. 11 graphs the drive frequency versus the scan time.In general the oscillating field can be applied along any of the x, y,or z Cartesian axes or any combinations of axes. Alternatively, themotion of the spins can be driven by modulating the pumping rate causedby the pumping light source.

In step 1006, a minimum or dip (or maximum/peak depending on theorientation of the oscillating field with respect the light propagationaxis) is observed when ω_(m) is close to the Larmor frequency ω₀ of thealkali metal atoms in the vapor cell, as illustrated in FIG. 11. Thespecific example of FIG. 11 shows a dip at about 31038 Hz, correspondingto a magnetic field with |B|=4434 nT.

Alternatively or additionally, to determine the resonance frequency withhigher precision a lock-in amplifier (LIA), as shown in FIG. 9, can beused. FIG. 12 illustrates the outputs (U: in-phase, V: quadrature, andphase) where the frequency reference of the amplifier is the firstharmonic of the LO_x. In the illustrated embodiment, the phase isrelated to the mismatch between the frequency ω_(m) of LO_x and theLarmor frequency ω₀ and magnetic resonance linewidth ΔB by Equation 3.

$\begin{matrix}{\Theta = {\arctan \left( \frac{\omega_{m} - \omega_{0}}{\gamma \; \Delta \; B} \right)}} & \left. 3 \right)\end{matrix}$

In the regime arctan

$\left( \frac{\omega_{m} - \omega_{0}}{\gamma \; \Delta \; B} \right) \approx \left( \frac{\omega_{m} - \omega_{0}}{\gamma \; \Delta \; B} \right)$

the phase Θ, and knowledge of ω_(m) and ΔB, can then be used todetermine ω₀ which, using Equation 2, provides an estimate of |B|.

Once the absolute value of the B-field has been estimated, the vectorcomponents, B_(x), B_(y), and B_(z), of the ambient background magneticfield can be estimated using a magnetometer operating in the non-SERFvector mode. Oscillatory fields, B_(i)(t)=β_(i) cos(ω_(i)t)Î, can beused along axes i=x, y, z, with amplitude β_(I) and frequency ω_(i).Consider the oscillatory field along the z axis: B_(z)(t)=β_(z)cos(ω_(z)t){circumflex over (z)}.

${{From}\mspace{14mu} {Equation}\mspace{14mu} 2},{\omega_{0} = {{\gamma {B}} = {\gamma \sqrt{\begin{matrix}\begin{matrix}{B_{x}^{2} + B_{y}^{2} + B_{z}^{2} +} \\{{2\; B_{z}\beta_{z}{\cos \left( {\omega_{z}t} \right)}} +}\end{matrix} \\\left. \left( {\beta_{z}{\cos \left( {\omega_{z}t} \right)}} \right) \right)^{2}\end{matrix}}}}}$

Using |B₀| to denote |B₀|=√{square root over (B_(x) ²+B_(y) ²+B_(z) ²)}and assuming

${\frac{\beta_{z}}{B_{0}}\text{<<}1},$

to first order the phase of the LIA output (see Equation 3) contains anoscillating component at the first harmonic of ω_(z) whose amplitude isrelated to B_(z):

$\Theta \approx {\frac{\beta_{z}}{{B_{0}}\Delta \; B}B_{z}{\cos \left( {\omega_{z}t} \right)}}$

assuming ω_(m)=γ|B₀|.

In the general case,

$\Theta \approx {{\frac{\beta_{z}}{{B_{0}}\Delta \; B}B_{z}{\cos \left( {\omega_{z}t} \right)}} + {\frac{\beta_{x}}{{B_{0}}\Delta \; B}B_{x}{\cos \left( {\omega_{x}t} \right)}} + {\frac{\beta_{y}}{{B_{0}}\Delta \; B}B_{y}{\cos \left( {\omega_{y}t} \right)}}}$

Thus, to obtain each of the cartesian components B_(x), B_(y), B_(z) ofthe ambient background magnetic field three different lock-in-amplifierscan be used with each referenced to the appropriate frequency ω_(x),ω_(y), ω_(z), respectively, as illustrated in FIG. 13. FIG. 13illustrates one embodiment of a portion of a magnetic field measurementsystem 1340 with a computing device 1350 (designated as the “OPM controlelectronics”), a magnetometer 1360, and a magnetic field generator 1362disposed around the magnetometer. The magnetic field generator 1362includes three pairs of coils (labeled Bx, By, and Bz) and arranged toproduce three fields B_(x), B_(y), and B_(z). The embodiment illustratedin FIG. 13 has three local oscillators (LO_x, LO_y, LO_z) 1382 a, 1382b, 1382 c applied to the coils Bx, By, B_(z), respectively, so that thecoils can generate an RF field with a frequency ω_(x), ω_(y), ω_(z),respectively, that can be swept. The embodiment illustrated in FIG. 13also includes three lock-in amplifiers (LIA_x. LIA_y, LIA_z) 1384 a,1384 b, 1384 c that each receive input from the detector of themagnetometer 1360 and the respective local oscillator 1382 a, 1382 b,1382 c and can be used to provide in-phase (“U”), quadrature (“V”), andphase angle (“θ”) outputs based on the inputs.

For instance, to retrieve B_(z), LIA_z in FIG. 13 produces in-phaseoutput given by V_(LIABz)

$\begin{matrix}{V_{LIABz} \approx {\frac{\beta_{z}}{{B_{0}}\Delta \; B}B_{z}}} & \left. 4 \right)\end{matrix}$

From Equation 4 and calibration of the ratio

$\frac{\beta_{z}}{B_{0}},$

B_(z) can be estimated from the phase output of LIA_z.

In step 1008, the magnetic field is modulated along two of the axes and,in step 1010, the response to the modulation is observed to obtainestimates of the ambient background magnetic field along the two axes.In step 1012, measurement the third vector component, Bx in this case,can be achieved by introducing a third oscillating field along the xaxis.

In at least some embodiments, the resolution in the measurement of avector component of the magnetic field is limited by the intrinsicsensitivity (spectral density) of the scalar OPM, δB_(s), and the ratio

$\frac{\beta_{v}}{B_{0}},$

thus the resolution δB_(v) of the measurement of a vector component ofthe field B in a bandwidth BW_(s) is equal to

${\Delta B_{v}} = {\delta \; B_{s}\sqrt{BW_{s}} \times {\frac{B_{0}}{\beta_{v}}.}}$

As indicated in steps 506 and 508 in FIG. 5, the compensation field canbe updated by measuring the reduced ambient background magnetic fieldusing a SERF mode OPM. These steps can be utilized in conjunction withthe methods illustrated in FIGS. 8 and 10. As an example, the magneticfield generators can apply magnetic field components Bx′, By′, Bz′, asdetermined using the method in any one of FIG. 4, 5, 8, or 10. Then asinusoidal field is applied along y and z, to retrieve, using the SERFmode OPM, the residual vector components of the reduced ambientbackground magnetic field which, in at least some embodiments, is at aresolution beyond ΔB_(v). The compensation field can then be updated toB′=x*(Bx′+ΔBx′)+y*(By′+ΔBy′)+z*(Bz′+ΔBz′).

A magnetic field measurement system may include an array ofmagnetometers with each magnetometer (or each set or pair ofmagnetometers) separately operating according to any one of the methodsillustrated in FIG. 4, 5, 8 or 10 or any similar method. Such an arrayallows for the detection of neural signals (or other signals ofinterest) over a region, such as the head of a user in the case of MEG.Each magnetometer (or each set or pair of magnetometers) may operateindividually or may information regarding the ambient backgroundmagnetic field may be shared for use with multiple magnetometers (orsets or pairs of magnetometers).

In at least some embodiments, a combined i) SERF mode and ii) scalar ornon-SERF vector mode magnetometer in a single device can provide highsensitivity, high dynamic range when combined with active shielding. Inat least some embodiments, a SERF mode magnetometer and scalar or vectormode magnetometer utilizing the same vapor cell can provide similarresults. In other embodiments, the two magnetometers are separate fromeach other, but are preferably located close together. In at least someembodiments, a combined i) SERF mode and ii) scalar or non-SERF vectormode magnetometer can be used with auxiliary sensors. Some embodimentsmay also include passive shielding or partial passive shielding and orflux concentrators as described in U.S. Provisional Patent ApplicationNo. 62/719,928, incorporated herein by reference in its entirety.

In at least some embodiments, the system allows the magneticcompensation field to match exactly (within the sensitivity of thescalar or non-SERF vector mode magnetometer) to the field at the vaporcell. In at least some embodiments, the system allows for a very smalldevice compared to using another high-dynamic range method. In at leastsome embodiments, the system allows for very fast and accuratefinite-field measurements compared to using another high-bandwidthmagnetometer. In at least some embodiments, the system can be realizedas a magnetometer or gradiometer or both. In at least some embodiments,the system when combined with auxiliary sensors allows for fast andaccurate measurements.

It will be understood that each block of the flowchart illustrations,and combinations of blocks in the flowchart illustrations and methodsdisclosed herein, can be implemented by computer program instructions.These program instructions may be provided to a processor to produce amachine, such that the instructions, which execute on the processor,create means for implementing the actions specified in the flowchartblock or blocks disclosed herein. The computer program instructions maybe executed by a processor to cause a series of operational steps to beperformed by the processor to produce a computer implemented process.The computer program instructions may also cause at least some of theoperational steps to be performed in parallel. Moreover, some of thesteps may also be performed across more than one processor, such asmight arise in a multi-processor computer system. In addition, one ormore processes may also be performed concurrently with other processes,or even in a different sequence than illustrated without departing fromthe scope or spirit of the invention.

The computer program instructions can be stored on any suitablecomputer-readable medium including, but not limited to, RAM, ROM,EEPROM, flash memory or other memory technology, CD-ROM, digitalversatile disks (“DVD”) or other optical storage, magnetic cassettes,magnetic tape, magnetic disk storage or other magnetic storage devices,or any other medium which can be used to store the desired informationand which can be accessed by a computing device.

The above specification provides a description of the invention and itsmanufacture and use. Since many embodiments of the invention can be madewithout departing from the spirit and scope of the invention, theinvention also resides in the claims hereinafter appended.

What is claimed as new and desired to be protected by Letters Patent ofthe United States is:
 1. A magnetic field measurement system,comprising: at least one first magnetometer configured for operation ina first mode selected from a scalar mode or a vector mode; at least onesecond magnetometer configured for operation in a spin exchangerelaxation free (SERF) mode which is different from the first mode,wherein each of the at least one first magnetometer and each of the atleast one second magnetometer comprises a vapor cell and at least one ofthe at least one first magnetometer and at least one of the at least onesecond magnetometer share the same vapor cell; at least one magneticfield generator; and a processor coupled to the at least one firstmagnetometer, the at least one second magnetometer, and the at least onemagnetic field generator and configured to: measure an ambientbackground magnetic field using at least one of the at least one firstmagnetometer operating in the first mode; generate a compensation fieldby the at least one magnetic field generator based on the measurementfrom the at least one first magnetometer;
 2. The magnetic fieldmeasurement system of claim 1, wherein the processor is furtherconfigured to: measure the ambient background magnetic field, reduced bycompensation field, using at least one of the at least one secondmagnetometer operating in the SERF mode; and update, in response to themeasurement of the ambient background magnetic field reduced bycompensation field, the compensation field using the at least onemagnetic field generator.
 3. The magnetic field measurement system ofclaim 1, wherein measuring the ambient background magnetic fieldcomprises, for each of two or three orthogonal axes: applying a firstmagnetic field along the axis; sweeping a frequency of the firstmagnetic field; measuring responses by the at least one of the at leastone first magnetometer in the first mode during the sweeping; anddetermining a vector component of the ambient background magnetic fieldalong the axis by observing a maximum or minimum in the responses orfitting the responses to a Lorentzian function.
 4. The magnetic fieldmeasurement system of claim 1, wherein measuring the ambient backgroundmagnetic field comprises: applying a first magnetic field along a firstaxis; sweeping a frequency of the first magnetic field; measuringresponses by the at least one of the at least one first magnetometer inthe first mode during the sweeping; and determining a magnitude of theambient background magnetic field along the axis by observing a maximumor minimum in the responses or fitting the responses to a Lorentzianfunction.
 5. The magnetic field measurement system of claim 4, whereinmeasuring the ambient background magnetic field further comprises, foreach of two or three orthogonal axes: applying a first magnetic fieldalong the axis; modulating the first magnetic field; measuring responsesby the at least one of the at least one first or second magnetometer ina vector mode during the modulation; and determining a vector componentof the ambient background magnetic field along the axis by observing theresponses to the modulated first magnetic field.
 6. The magnetic fieldmeasurement system of claim 1, further comprising at least one localoscillator coupled to at least one of the at least one magnetic fieldgenerator; and at least one lock-in amplifier, each of the at least onelock-in amplifier coupled to a one of the at least one local oscillatorand at least one of the at least one first magnetometer.
 7. A method ofoperating the magnetic field measurement of claim 1, the methodcomprising: measuring an ambient background magnetic field using atleast one of the at least one first magnetometer; and generating acompensation field by the at least one magnetic field generator based onthe measurement from the at least one first magnetometer.
 8. The methodof claim 7, further comprising measuring the ambient background magneticfield, reduced by compensation field, using at least one of the at leastone second magnetometer; and updating, in response to the measurement ofthe ambient background magnetic field reduced by compensation field, thecompensation field using the at least one magnetic field generator. 9.The method of claim 7, wherein measuring the ambient background magneticfield comprises, for each of two or three orthogonal axes: applying afirst magnetic field along the axis; sweeping a frequency of the firstmagnetic field; measuring responses by the at least one of the at leastone first magnetometer in the first mode during the sweeping; anddetermining a vector component of the ambient background magnetic fieldalong the axis by observing a maximum or minimum in the responses orfitting the responses to a Lorentzian function.
 10. The method of claim7, wherein measuring the ambient background magnetic field comprises:applying a first magnetic field along a first axis; sweeping a frequencyof the first magnetic field; measuring responses by the at least one ofthe at least one first magnetometer in the first mode during thesweeping; and determining a magnitude of the ambient background magneticfield along the axis by observing a maximum or minimum in the responsesor fitting the responses to a Lorentzian function.
 11. The method ofclaim 10, wherein measuring the ambient background magnetic fieldfurther comprises, for each of two or three orthogonal axes: applying afirst magnetic field along the axis; modulating the first magneticfield; measuring responses by the at least one of the at least one firstor second magnetometer in a vector mode during the modulation; anddetermining a vector component of the ambient background magnetic fieldalong the axis by observing the responses to the modulated firstmagnetic field.
 12. The method of claim 7, wherein the magnetic fieldmeasurement system further comprises at least one local oscillatorcoupled to at least one of the at least one magnetic field generator;and at least one lock-in amplifier, each of the at least one lock-inamplifier coupled to a one of the at least one local oscillator and atleast one of the at least one first magnetometer.
 13. A magnetic fieldmeasurement system, comprising: a magnetometer; at least one magneticfield generator; a processor coupled to the magnetometer and the atleast one magnetic field generator and configured to: i) measure anambient background magnetic field using the magnetometer in a first modeselected from a scalar mode or a vector mode; ii) generate, in responseto the measurement of the ambient background magnetic field, acompensation field using the at least one magnetic field generator; iii)measure a target magnetic field using the magnetometer in a spinexchange relaxation free (SERF) mode which is different from the firstmode; and iv) determine when the magnetometer is not operating in SERFmode and automatically perform steps i) and ii) again.
 14. The magneticfield measurement system of claim 13, wherein the processor is furtherconfigured to: measure the ambient background magnetic field, reduced bycompensation field, using the magnetometer in the SERF mode; and update,in response to the measurement of the ambient background magnetic fieldreduced by compensation field, the compensation field using the at leastone magnetic field generator.
 15. The magnetic field measurement systemof claim 13, wherein measuring the ambient background magnetic fieldcomprises, for each of two or three orthogonal axes: applying a firstmagnetic field along the axis; sweeping a frequency of the firstmagnetic field; measuring responses by the magnetometer in the firstmode during the sweeping; and determining a vector component of theambient background magnetic field along the axis by observing a maximumor minimum in the responses or fitting the responses to a Lorentzianfunction.
 16. The magnetic field measurement system of claim 13, whereinmeasuring the ambient background magnetic field comprises: applying afirst magnetic field along a first axis; sweeping a frequency of thefirst magnetic field; measuring responses by the magnetometer in thefirst mode during the sweeping; and determining a magnitude of theambient background magnetic field along the axis by observing a maximumor minimum in the responses or fitting the responses to a Lorentzianfunction.
 17. The magnetic field measurement system of claim 13, furthercomprising at least one local oscillator coupled to the at least onemagnetic field generator; and at least one lock-in amplifier, each ofthe at least one lock-in amplifier coupled to a one of the at least onelocal oscillator and the magnetometer.
 18. The magnetic fieldmeasurement system of claim 13, wherein the processor is furtherconfigured to repeat steps i) and ii) periodically at a selectedrepetition rate.
 19. The magnetic field measurement system of claim 13,wherein the processor is further configured to repeat steps i) and ii)aperiodically.
 20. A method of operating the magnetic field measurementsystem of claim 13, the method comprising: i) measuring the ambientbackground magnetic field using the magnetometer operating in the firstmode selected from the scalar mode or the vector mode; ii) generating,in response to the measurement of the ambient background magnetic field,the compensation field using the at least one magnetic field generator;and iii) measuring the target magnetic field using the magnetometeroperating in the spin exchange relaxation free (SERF) mode which isdifferent from the first mode and iv) determining when the magnetometeris not operating in SERF mode and performing steps i) and ii) again.